Apparatus for suppression of backward wave oscillation in traveling wave tubes having bifilar helical wave structure



Oct. 11, 1966 D. J. BLATTNER 278,792 FOR SUPPRESSION OF BACKWARD W APPARATUS AVE OSCI TION IN TRAVELING WAVE TUBES HAVING BIF'ILAR HELI WAVE STRUGT Fi Sept. 9. 1

BEAM VOLTAGE V i F5 F0 FREQUENCY SIGNAL 7 INVENTOR.

F] -4 DONALD J. BLATTNER ATT RNEYS United States Patent APPARATUS FOR SUPPRESSION F BACKWARD WAVE OSCILLATION IN TRAVELING WAVE TUBES HAVING BIFILAR HELICAL WAVE STRUCTURE Donald J. Blattner, Princeton, N.J., assignor' to the United States of America as represented by the Secretary of the Air Force Filed Sept. 9, 1965, Ser. No. 488,834 1 Claim. (Cl. 315-3.6)

This application is a continuation-in-part of my copending application Serial No. 42,453, filed July 12, 1960, now abandoned.

The present invention relates to a traveling-wave tube and more particularly to a traveling-wave amplifier tube having a bifilar helix radio frequency circuit which is capable of virtually eliminating backward-wave oscillations.

A bifilar helix radio frequency circuit has some advantages over a single wire helix in traveling-wave tube amplifiers. In particular, by applying different static potentials to the two wires, the bifilar helix can be used to focus the electron beam by electrostatic means, obviating the need for a magnetic field.

Radio frequency propagation on the bifilar helix can occur in either of two modes. One of these is a symmetric mode, which has just the field distribution commonly used in unifilar helix traveling-wave tubes. This mode has even-numbered space harmonics. The second possible mode of propagation is an antisymmetric mode which has odd-numbered space harmonics. The space harmonic of the antisymmetric mode can couple strongly to the electron beam, producing backward-wave oscillations which are wholly undesirable in a tube designed as an amplifier because of the losses in gain and power output.

It is thus an object of this invention to provide a traveling-wave tube which has optimum gain and output power characteristics.

It is another object of this invention to provide a traveling-wave tube which contains structure that virtually eliminates backward-wave oscillations.

It is a further object of this invention to provide a traveling-wave tube having a bifilar helix radio frequency circuit with means uniformly spaced along the entire length of the bifilar helix for electrically connecting the helixes.

Other objects, novel features, and advantages of this invention will become apparent upon consideration of the embodiments illustrated in the accompanying drawings and hereinafter described.

In the drawings:

FIGURE 1 is a graph illustrating the relationship in a bifilar helix traveling-wave tube between beam voltage and frequency under symmetrical and antisymmetrical modes;

FIGURE 2 is a cross-sectional view, partially schematic, of a first embodiment of the traveling-wave tube;

FIGURE 3 is a schematic view showing a bifilar helix construction in a second embodiment of the invention; and

FIGURE 4 is a schematic view showing a bifilar helix construction in a third embodiment of the invention.

Referring now more particularly to FIGURE 1, the beam voltage required for synchronism with the m=0 space harmonic wave of the symmetric mode, which is the usual traveling-wave tube wave, or the m=l space harmonic wave of the antisymmetric mode, is a function of frequency, 1, expressed as 3,2 79 Patented Oct. 11, 1966 21raf/c where a is the helix diameter and c is the velocity of light along the helix wire. The tube can oscillate at frequency F when the beam voltage on the circuit is set for amplification of frequency F This oscillation is selfexcited, and does not involve reflections from the ends of the circuit of discontinuity. It requires only that the beam current be greater than some minimum value which depends upon the beam and circuit geometry, circu t length, and circuit loss. Once this starting current is reached, the power in the oscillation rises quickly with further increase of beam current. The modulation of the beam at the oscillation frequency cuts down the gain and output power of the signal which was fed into the amplifier. Thus the amplifier performance can be degraded by the backward-wave oscillations.

The apparatus shown in the cross-sectional, partially schematic, view of FIGURE 2 comprises a traveling-wave tube 10 which contains structure that alleviates the backward-wave oscillation problem. The tube 10 includes an electron gun 12 for producing and directing an electron stream along a reference path generally indicated by a dash line 14 and a collector electrode 16 at the opposite end of tube 10 where the reference path terminates. The electron gun section 12 of the traveling-wave tube is made up of an electron emissive cathode 18, a focusing electrode 20, and an accelerator electrode 22. The focusing electrode 20 and accelerating electrode 22 are mounted in parallel spaced relation to the cathode 18 in the order named. Both electrodes 20 and 22 include centrally located apertures for passage of the electron stream. A wave-transmission line 24 is disposed between accelerating electrode 22 and collector16 and encompass ing the reference path 14. The transmission line 24 includes first and second helical conductors 26 and 28 which are wound in bifilar fashion. The windings 26 and 28 of the line 24 are coupled to a radio frequency signal source 29 at the end of the line adjacent to the electron gun 12 and to a load 30 at the opposite end of the tube 10. The windings may be electrically intercoupled if desired at the signal source input by capacitor 31 and to the load by capacitor 32 mounted within the traveling-wave tube 10. Means 33 are uniformly spaced along the entire length of the windings 26 and 28 for electrically connecting the windings.

The means for electrically connecting the windings along the length of the transmission line virtually eliminate the backward-wave oscillation by making it impossible for the antisymmetric mode to exist. The antisymmetric mode cannot exist if the opposing wires of the two helixes in any given Z-plane are constrained to have the same radio frequency voltage. Such constraint can be applied by strapping the helixes, as shown in FIGURES 2, 3 and 4. In FIGURE 2 diametrical strapping bars 33 electrically connect the two helixes so that the radio frequency potential is held symmetrical. This type of strapping also ties the two helixes together at a common static voltage which makes it impossible to focus the stream of electrons by maintaining different static voltages on the two helixes. The inclusion of radio frequency bypass condenser 34 in the diametrical strapping bar, as shown in FIGURE 3, makes it possible to focus the stream of electrons 14 by maintaining different static voltages on the two helixes. FIGURE 4 shows a third means for electrically connecting windings 26 and 28. A plurality of narrow conductive rings 36 are placed around the windings 26 and 28 which couple capacitively to the wires of the windings on the opposite sides. This third method has the advantage of keeping the strapping structure out of the electron stream and out of the static focusing field.

A possible external circuit for the traveling-wave tube is indicated in FIGURE 2 for a more complete understanding of the invention. The electronic emissive cathode 1-8 is connected to a source of reference potential, here illustrated as ground. The focusing electrode 20 is connected to the negative terminal of a first source of operating potential 38 and accelerating electrode 22 is connected to the positive terminal of the source 38. An intermediate point in the source 38 is grounded. The signal source 29 is so connected as to apply a positive D.C. voltage to the transmission line winding 28. Similarly, the load circuit 30 is so connected as to apply a positive D.C. voltage to the transmission line winding 26. A second source of positive potential 40 is electrically connected to collector 16.

In the operation of traveling-wave tube 10, when the tube is rendered operative, a radio frequency signal is applied to wave-transmission line 24 from source 29. A stream of electrons are emitted from the cathode 18 and are focused by passing the stream through the aperture in the focusing electrode 20. The electrons are then accelerated by means of their passage through the aperture in the accelerating electrode 22. The electrons of the stream continue along the reference path 14 and are collected by collecting electrode 16. The velocity of the stream of electrons passing along reference path 14 is adjusted so that it is approximately equal to the wave propagation velocity of the signal wave traveling along transmission line 24. As the electrons enter that portion of reference path 14, which is encompassed by transmission line 24, they are subject to an electric field established :by the application of radio frequency signals from source 29 to the transmission line. The electrons in the stream of electrons are continuously accelerated or decelerated along the reference path 14, because the wave-propagation and electron velocities are equal. The electrons tend to bunch as they move along the reference path, and as the stream of electrons moves toward collector 16 it imparts energy to the traveling signal field. The otherwise present backward-wave oscillations are eliminated by the and 36. Where the strapping bar embodiment of FIG- URE 2 is used, magnetic focusing means (not shown) which are conventional in the art must be installed along the length of the traveling-wave tube 10. For a complete discussion of the uses and mechanics of traveling-wave tubes, reference is made to Traveling-Wave Tubes, 'by

I. R. Pierce, published by D. Van Nostrand, Inc., New

York, 1950.

The invention is not intended to :be limited to the examples of embodiments shown and described, but may on the contrary, be capable of many modifications without departing from the spirit of the invention.

.1 claim:

A traveling-wave tube comprising: a wave transmission line including two identical coaxial coextensive conductive helices with a relative axial displacement of half their pitch, means for producing and directing an electron stream alongthe axis of said helices, means for applying a radio frequency signal to said helices for establishing a traveling signal wave, and a plurality of conductive rings concentric with the axis of said helices having inner diameters slightly greater than the outer diameters of said helices and spaced along said helices at intervals of half the pitch of said helices, whereby at half-pitch intervals a coupling having low impedance at high frequencies is estlablished between diametrically opposite points on said he ices.

References Cited by the Examiner UNITED STATES PATENTS 8/1959 Webber et a1 31-5-3.5 5/1960 Birdsall 3153.5

GEORGE N. WESTBY, V. LAFRANCHI,

Assistant Examiners. 

